The present invention relates to a process for obtaining potassium chloride relatively free of fluoride ion contamination by selective solubility in water having a pH within a narrow range close to neutral.
Potassium chloride as obtained as a by-product of chemical processes and from other sources is often contaminated with small amounts of fluoride ion. This contaminant is deleterious for many applications of potassium chloride and it must be reduced to an acceptable level before use. These mixtures are, further, typically somewhat acidic, having pH values below about 6 when dissolved in water. It is known from Japanese Patent Application JP 53 033995 that some of the potassium fluoride can be removed from potassium chloride solids by extraction with a limited amount of water. Potassium fluoride is, however, not removed as completely as is desirable and considerably more potassium chloride is lost than is desirable using the method as disclosed when the potassium chloride contains some acid contaminants as well as the potassium fluoride. It is further known to separate certain contaminating salts, such as sodium chloride, from potassium chloride by fractional crystallization.
Improved processes for removing fluoride ion from potassium chloride would be of considerable interest.